Question: Why is Autocorrelation giving negative values?

I am tyring to simulate the intensity autocorrelation of a femtosecond laser pulse.   The pulse intensity is set up as a 4096 point Vector of real values using a sech function for the pulse shape (see below). 

Being that it's the intensity, not the complex field, of the laser pulse, it is always real-valued and positive.    Therefore, the autocorrelation of the pulse intensity should be positive at all points in time.   But in fact, when I use the AutoCorrelation command on this Vector, it results many negative-valued points (see below).

What is going on?   Why am I getting negative values?



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